1. Field of the Invention
This invention relates to a process for producing .alpha.-olefin polymers. More particularly it relates to a process for producing .alpha.-olefin polymers having a high crystallinity and a good particular form with a high yield, by the use of a novel catalyst suitable for .alpha.-olefin polymerization, particularly gas phase polymerization and further a combination of slurry or bulk polymerization with gas phase polymerization as a modification of gas phase polymerization.
2. Description of the Prior Art
It is well known that .alpha.-olefins are polymerized by means of the so-called Ziegler-Natta catalysts consisting of a compound of metals of Groups IV-VI of the Periodic Table and an organometallic compound of metals of Groups I.about.III of the Table, including those modified by adding an electron donor, etc. Among the above transition metal compounds as a component of the catalysts, titanium trichloride has been most broadly used for obtaining highly crystalline polymers such as those of propylene, butene-1, etc. Such titanium trichloride is classified into the following three kinds according to its preparation:
(1) a product obtained by reducing TiCl.sub.4 with hydrogen followed by milling for activation by means of a ball mill (which is referred to as titanium trichloride (HA));
(2) a product obtained by reducing TiCl.sub.4 with metallic aluminum, followed by ball milling for activation, i.e. a compound expressed by the general formula TiCl.sub.3.1/3AlCl.sub.3 (which is the so-called titanium trichloride (AA)); and
(3) a product obtained by reducing TiCl.sub.4 with an organoaluminum compound followed by heat treatment.
However, since any of these kinds of titanium trichloride are not fully satisfactory, various improvements have been considered and tried. As one of them, a process has been proposed wherein the titanium trichloride obtained by reducing TiCl.sub.4 with an organoaluminum compound is further treated with an electron donor and TiCl.sub.4 to thereby raise its catalyst activity and also reduce the amount of an amorphous polymer formed (e.g. Japanese patent application laid-open No. Sho 47-34478/1972). However, such a process has a drawback in that the resulting catalyst lacks heat-stability.
Further, another process has also been proposed wherein TiCl.sub.4 and an organoaluminum compound are respectively and separately mixed and reacted with a definite amount of a complex-forming agent (electron donors being one kind thereof), followed by mixing and reacting together the resulting two reaction liquids to prepare a solid catalyst component (Japanese patent application laid-open No. Sho 53-9296/1978). However, this process also has a drawback in that the resulting catalyst lacks heat stability, as in the case of the above Japanese patent application laid-open No. Sho 47-34478/1972).
Furthermore, there have been proposed a process of adding to TiCl.sub.4, a uniform liquid material consisting of an organoaluminum compound and an ether or inverting this addition order to prepare a liquid product containing titanium trichloride (Japanese patent application laid-open No. Sho 52-115797/1977), and a process of further heating the above liquid product to 150.degree. C. or lower to deposit finely particular titanium trichloride (Japanese patent application laid-open No. Sho 52-47594/1977), but these processes also have a drawback in that the resulting catalysts lack heat stability.
On the other hand, as for the phase in which the polymerization processes using Ziegler-Natta catalysts are carried out, slurry polymerization carried out in a solvent such as n-hexane (e.g. Japanese patent publication No. Sho 32-10596/1957), bulk polymerization carried out in a liquefied monomer such as liquefied propylene (e.g. Japanese patent publications No. Sho 36-6686/1961 and No. Sho 38-14041), and gas phase polymerization carried out in a gaseous monomer such as gaseous propylene (e.g. Japanese patent publication Nos. Sho 39-14812/1964 and Sho 42-17487/1964) have been well known, and further a process of bulk polymerization followed by gas phase polymerization has also been known (e.g. Japanese patent publication No. Sho 49-14862/1974 and Japanese patent application laid-open No. Sho 51-135987/1973). Among these processes, the gas phase polymerization process has advantages in that neither recovery and reuse of solvent nor those of liquefied monomer such as liquefied propylene, used in polymerization are necessary, whereby the recovery cost of solvent or monomer is slight and it is possible to simplify equipments for producing .alpha.-olefin polymers. Gas phase polymerization process, however, have drawbacks in that since the monomer inside the polymerization vessel is present in gas phase, the monomer concentration is so low as compared with those in the slurry or bulk polymerization, that the reaction rate is so slow; hence the retention time must be increased to raise the polymer yield per unit weight of the catalyst, which, however, results in enlargement of the reactor, or a modified trialkylaluminum must be used to raise the catalyst activity, which, however, results in reduction of the stereoregularity of the resulting polymer. Further in the case of gas phase polymerization process, polymer particles are liable to become ununiform due to the ununiform catalyst particles. This is liable to cause cohesion of polymer particles or clogging at the discharge port of the polymerization vessel or on the transportation line, resulting in difficulty in a long term stabilized continuous operation and a large dispersion of the quality.
The present inventors previously invented a polymerization process having no drawbacks as above even in the case of gas phase polymerization, i.e. a process for producing .alpha.-olefin polymers wherein .alpha.-olefins are polymerized in the presence of a catalyst prepared by reacting TiCl.sub.4 with a reaction product of an organoaluminum compound with an electron donor, reacting the resulting solid product with an electron donor and an electron acceptor and combining the resulting solid product with an organoaluminum compound (Japanese patent application No. Sho 55-12875/1980, which will hereinafter be referred to as the prior invention). This prior invention has been characterized in that a long term, stabilized operation of gas phase polymerization is possible; the shelf stability of the catalyst and its heat stability during polymerization are high; the polymer yield by way of gas phase polymerization amounts to 5,000 to 6,000 g/g of solid product; and the amount of amorphous polymer formed is reduced. However, the shelf stability of the catalyst at high temperatures of 35.degree. C. or higher has been insufficient, and further a catalyst having a higher heat stability during the polymerization has been required.
Thus, the object of the present invention is to provide a process for producing .alpha.-olefin polymers by the use of a catalyst having an improved shelf stability and heat stability, which, even when used in gas phase polymerization, slurry polymerization or bulk polymerization, has a high catalyst activity; can yield a polymer having a uniform particle size, with a low percentage of amorphous polymer formed; and particularly can exhibit the advantages of gas phase polymerization.
The present inventors have further continued studies for improvement and have found that the shelf stability and heat stability of the catalyst can be further improved by subjecting the solid obtained by reacting TiCl.sub.4 with a reaction product of an organoaluminum compound with an electron donor, to a polymerization treatment with an .alpha.-olefin, followed by reaction with an electron donor and an electron acceptor, and have attained the present invention.